1. Field of the Invention
The present invention relates to a focus servo device which is suitable for controlling the focusing operation of an optical beam which is outputted from an optical head in the recording apparatus and reproducing apparatus corresponding to a magneto-optical disc.
2. Description of Related Art
In a recording apparatus and reproducing apparatus corresponding to a disc type recording medium such as optical disc and magneto-optical disc or the like, the optical beam outputted from the optical head must be controlled to obtain adequate focusing condition on a disc recording surface and a focus servo device is provided, for this purpose, to drive an objective lens in the optical head either towards or away from the recording surface of a disc.
Since the focus servo pull-in range is rather narrow, when the recording or reproducing operation is activated and focus servo is deviated by vibration or the like, the objective lens position is shifted up to the focus pull-in range by executing the focus search operation and thereafter the focus servo loop is turned on to execute the focus servo. As the rising process for the recording or reproducing operation, the spindle servo and tracking servo are executed, upon execution of the focus search and focus servo loop close operation. When this rising process is completed, the scanning for recording or reproducing operation can be realized with the optical beam for recording or reproducing operation.
As an example of the focus search operation employed in the optical disc reproducing apparatus, the objective lens is shifted in the direction to be in contact with or isolated from the disc surface from the intermediate point between the furthest position and the nearest position from the disk surface.
As a focus error signal FE obtained by the arithmetic process of the output of a 4-split detector for detecting a reflected beam from a disc, a curve similar to the character S as shown in FIG. 1B can be obtained within the focus pull-in range. Moreover, as the RF signal (sum signal of 4-split detector), the signal as shown in FIG. 1A can be obtained. Here, a FOK signal can also be obtained as shown in FIG. 1C by comparing the RF signal with the predetermined threshold value Th, but this FOK signal indicates the focus pull-in range.
Since the maximal value of the RF signal shown in FIG. 1A can be obtained, for example, within the range of about 1 .mu.m to 200 .mu.m, the focus pull-in range (FOK signal) indicated on the basis of this range also indicates the corresponding range.
With the focus search operation, when the focus servo is turned on in such a phase that the objective lens position is controlled to the focus pull-in range equivalent to the H period of this FOK signal, the adequate focus control is executed. That is, in the focus pull-in range, the focus servo control is executed to control the objective lens for the calling point of the focus zero-cross detecting signal (FZC signal, namely focus on detecting signal) shown in FIG. 1D.
FIG. 2 is a cross-sectional view showing the concept of an optical disc and a magneto-optical disc. The cross-sectional view indicates a recording surface 1a on which an audio data or the like is recorded and the surface 1b of a protective layer formed, for example, of a transparent resin or the like. In such a disc 1, the recording surface 1a is formed as the laser beam reflecting surface to reflect the light beam as indicated by the arrow mark of solid line but it is also known that the disc surface 1b also reflects to a certain degree the light as indicated by the arrow mark of a broken line and its reflectivity is assumed as about 8%. The reflectivity of the recording surface la is different in the optical disc and magneto-optical disc and it is assumed as about 70 to 80% in the case of the optical disc or as about 15 to 25% in the case of the magneto-optical disc.
FIG. 3A to FIG. 3E respectively show a distance of the objective lens to the disc surface, FOK signal, RF signal, focus error signal FE, FZC signal corresponding to this distance.
Here, it is assumed that the focus search operation has been conducted while the objective lens is moved closer to the disc surface from a distant position.
As shown in FIG. 3A, when the objective lens 3a.sub.1 is located to the focusing position for the disc surface 1b, since the surface 1b also has the reflectivity as explained above, the RF signal can be outputted as shown in FIG. 3C from the surface of substrate 1b. When the threshold value Th for the RF signal is set to the level indicated in the figure, the FOK signal can also be obtained as shown in FIG. 3C from the substrate surface 1b.
Moreover, when the objective lens comes closer to the disc surface and it is focused to the recording surface 1a as shown by the lens 3a.sub.2, the FOK signal can be obtained on the basis of the RF signal as shown in FIGS. 3A to 3E.
Namely, since the disc surface has a certain reflectivity, the FOK signal indicting the focus pull-in range which should naturally be obtained only at the reflecting surface may also be obtained even when the focusing is obtained for the disc surface 1b.
In the case of an optical disc such as a compact disc, since the reflectivity of the recording surface 1a is about 70 to 80% and that of the disc surface 1b is about 8%, difference is very large and it can be avoided by setting a higher threshold value Th that the FOK signal is obtained by reflection at the disc surface 1b.
In the case of a magneto-optical disc, the reflectivity at the recording surface 1a is 15 to 25% which is not so much different from that of the disc surface 1b. Therefore, separation by the threshold value Th is difficult. Therefore, in the focus search operation, if the objective lens is assumed to be moved in the direction to become closer to the disc surface from the distant position, the focus pull-in range and focus-on condition are indicated when the lens is focused at the substrate surface before these are obtained at the reflecting surface. Namely, a false FOK signal can be obtained. Of course, if the servo loop is closed with such false FOK signal, focus control may be completed erroneously.
In order to eliminate such event, a method is employed that the objective lens is forcibly moved, in the recording/reproducing apparatus for a magneto-optical disc, to the nearest position to the disc surface in the focus search operation, thereafter the objective lens is moved, from such condition, to become far from the disc and at the time of this movement, the focus servo is turned on based on the FOK signal obtained initially. For example, the Japanese Laid-open Patent No. 61-158036 (Laid-open Date: Jul. 7, 1986).
However this method has a problem that a longer time is required for the focus search because the operation that objective lens must be once located nearer to the disc surface is necessary. As the initial process in the recording/reproducing apparatus, after the focus search and focus servo have been executed, the spindle servo and tracking servo are executed and when such initial process is completed, the recording or reproducing operation can be started. Namely, quick initializing process is required to realize the quick reproducing operation during the reproducing operation and after the access is made. For this purpose, it is the most important requirement to realize quick focus search operation which takes the longest time in the initializing operations. Accordingly, it is a serious problem that a longer time is taken by the focus search as explained above.